1. Field of the Invention
The instant invention is directed to an apparatus for controlling the thickness of extrudate moving from a die. Still more specifically the instant invention is directed to an adjusting means for controlling the thickness of a polymeric melt extruding from a slot die in which the adjustment means comprises a bolt having two separate threaded sections for precisely controlling the movement of a flexible lip, which with a stationary lip defines the die lip opening.
2. Background of the Prior Art
A common method for forming a plastic film is to extrude a polymer, which is in a liquid or semi-liquid state, through a shaping device. The shaping device through which the liquid or semi-liquid polymer is extruded is called a die. The extrudate, as the liquid is called, is then solidified by suitable means to produce the solid state polymeric film.
The two most common dies employed in the production of polymeric film are the circular die and the slot die. The circular die is a circular shaped opening through which polymeric liquid is extruded to form a tubular shaped film. The slot die is employed to produce a single ply film when solidified. It is this latter type of die to which the instant application is directed.
Slot dies are employed in the production of polymeric films where the liquid state polymer is in the form of a melt, that is, a homogeneous single component liquid or, alternatively, in the form of a solution, a two component liquid comprising the polymer, the solute, and solvent, in which the polymer is dissolved. In either case the thickness of the film produced is a function of the die lip opening. The die lip opening is the opening provided for the extrudate, the liquid or semi-liguid polymer in melt or solvent form, to exit the die. Although this thickness is critical in all applications, it is often of more critical importance when the extrudate is a melt because the final thickness of the formed film is proportional to the tickness of the liquid melt as it exists through the slit die lip opening. Indeed, all polymeric films which require precision gauge control are melt extruded through a slotted die. Included in the class of polymeric films requiring precision gauge control, which are formed by melt extrusion through a slotted die, are polyester, polyolefins, polycarbonate and polyacetal films.
In the production of films by melt extrusion through a slotted die, the hot polymeric melt enters the die body, which is usually constructed of steel, and extruded through the die lip opening as a thin sheet. Along the length of these lips are located adjusting bolts which are spaced equidistantly apart from each other. The bolts are turned with a wrench or other appropriate means to open and close the die lip opening to thus alter the thickness of the film which extrudates therethrough.
In the prior art the adjusting bolts, often called the die bolts, comprise a long threaded section which moves through a tapped hole in a stationary block connected to the die body. Adjacent to one end of this long threaded section is a head provided for the turning of the bolt by a suitable tool. At the other end of the long threaded section is a nonthreaded cylindrical section. Disposed over this non-threaded section is a clip, a forked shaped yoke integrally connected with a perpendicular element which is connected to the die lip. The non-threaded section of the prior art bolt is provided with a stop mechanism, a larger diameter cylindrical section which limits the movement of the yoke of the clip. An extension of the non-threaded cylindrical section beyond the enlarged diameter portion may be included to provide additional pushing force in the direction of closing the die lip.
The above design represents an operable method by which the die lips are adjusted to provide gauge control. However, certain disadvantages result from the operation of this design. Because the threaded section is relatively coarse, a necessity in view of the strength requirements, the sensitivity of the bolt is very great. Alternately stated, the resolution is very poor. That is, a relatively small turn of the bolt makes for a significant change in the die lip opening. This makes it necessary for the operator to be quite skilled in the operation of the bolts to achieve the desired change in thickness.
In addition to the above disadvantage, poor resolution, the employment of a single threaded section also results in high frictional torque that must be overcome. Thus a significant degree of force must be employed to turn the bolt. This, in reality, is another aspect of poor resolution. The more difficult it is to turn the bolt the more difficult it is to adjust the bolt to the desired degree so as to obtain the desired change in die lip opening.
Another disadvantage of the push-pull die bolt of the prior art relates to the non-threaded section over which the yoke of the clip is disposed. Again, the problem is one of poor resolution. This problem occurs when the yoke is disposed approximately intermediate between the ends of the non-threaded section defined by the threaded section at one end and the enlarged diameter portion of the non-threaded section at the other. With the bolt in this position, turning of the bolt does not actuate any movement of the die lips. This so-called "neutral zone" makes adjustment all the more difficult.
Another difficulty associated with the employment of the push-pull die bolt of the prior art again relates to the problem of high frictional torque. Although high frictional torque is caused in part by the employment of a single coarse threaded bolt, of equal consequence is the temperature to which the bolt is exposed. Typically, the die body is exposed to high polymeric melt temperatures which often exceed 500.degree. F. At these temperatures the typically employed lubricants for bolts, petroleum based lubricants, are ineffective. In the absence of a lubricant, high temperature causes the bolt threads to seize. This not only results in high frictional torque resistance but also causes frequent replacement of the bolt due to wear.
The above disadvantages of the push-pull die bolt of the prior art suggests the need of a new design which overcomes these difficulties while retaining the useful properties inherent in the use of a push-pull design.